Gas oil, a product from the petroleum refining process, contains paraffins. Since paraffins are solid at about room temperature, an increase in the paraffin content will reduce the fluidity of the gas oil at low temperatures as in winter. When a sample of petroleum product, such as gas oil or lubricating oil, is gradually cooled in a prescribed way, its paraffin content deposits or separates increasingly until the walls of the container holding the sample begins to appear cloudy. This point is known as a cloud point. The same phenomena occur with plasticizers and surfactants. Cloud point determination is important for the specifications of petroleum products in winter. There are many other cases in which the deposition of solids upon cooling of petroleum products must be determined.
Japanese Industrial Standards K-2269 prescribes testing procedures for the cloud point of petroleum products as well as the fluid point of crude oil and petroleum products. The cloud point testing procedure is briefly defined: "Place a test tube holding 45 ml of a sample into an outer tube in a cooling bath and cool it in a prescribed manner. Take out the test tube each time the sample temperature has dropped by 1.degree. C. The temperature at which a cloudy appearance has just been seen in the bottom of the sample shall be deemed a cloud point." Remarks say: "The use of an automatic cloud point tester instead is permissible provided it is confirmed beforehand in conformity with JIS Z-8402 that there is no significant difference between the result thereby attained and the result obtained by the testing procedure defined above." As the testing equipment are furnished: a test tube in the center of which a test thermometer is held vertically with a cork stopper in place, outer tubes each fitted with a bottom disk of cork or felt and adapted to house the test tube therein, and cooling baths each equipped with a thermometer for cooling solution. The test tube is able to be placed into and taken out of an outer tube through an annular gasket kept in close contact with the outer wall of the test tube. The annular gasket tightly fitted around the test tube is loosely in contact with the outer tube to support the test tube vertically therein. The testing procedure is specified to comprise:
(1) keeping a sample at a temperature higher than a predicted cloud point by at least 14.degree. C.;
(2) pouring the sample into the test tube up to the height of a marked line (at the middle);
(3) hermetically sealing the test tube with the cork stopper fitted with a thermometer, in such manner that the thermometer is held upright in the center, with its tip in contact with the bottom of the tube;
(4) placing the test tube together with the annular gasket into the outer tube fitted with a bottom disk;
(5) placing the outer tube, together with the test tube housed therein, into a first cooling bath and supporting the outer tube lest it float 25 mm or more from the surface of the cooling solution;
(6) after the sample temperature has reached the vicinity of the predicted cloud point, taking out the test tube upon each drop by .degree. C., rapidly and without causing any sample vibration, inspecting the sample for any cloudy appearance seen in the bottom, and then placing the test tube back into the outer tube; and
(7) repeating these steps and recording the reading of the thermometer at the moment when visible clouding or haze appears for the first time in the bottom of the sample. (In case when clouding is not observed with a sample that has been cooled down to 10.degree. C., the test tube shall be transferred to an outer tube in a second cooling bath at a lower temperature. If clouding is still not observed at a sample temperature of -7.degree. C., the test tube shall be transferred to an outer tube in a third cooling bath at a much lower temperature.)
Thus the procedure requires bothersome handling; repeatedly placing the test tube into the outer tube taking it out carefully by hand.
The United States too has similar provisions.
The automatic cloud point tester used there, for petroleum products, is such that a container holding about 50 cc of a sample is placed in a cooling tank, cooled at a cooling rate of 2.degree. C./min, and deposition of paraffins is detected by a light absorption or diffusion method.
That automatic cloud point testing method faithfully follows the principle of cloud point determination. Actually, the sampling container has too large a capacity to ensure at all times the uniformity of temperature of the substance being tested. The measurement time is prolonged because the cooling from room temperature down to -20.degree. C. takes much time. Moreover, the sample must be introduced each time into a large container, and this sampling is a troublesome work requiring both time and labor. The detection by the light absorption or diffusion method that depends on a certain amount of deposit for judgment lacks accuracy. Each run for cloud point determination takes a period of about 60 minutes.
The object of the this invention is to develop a novel technique for accurately determining the cloud points of petroleum products in a simpler and quicker way than heretofore.